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A quick recovery Electric heater is rated at 4500 W and operated at 120 V, What will be the rating of circuit breaker to be used to protect it?

A 60 amperes

The ampere current drawn by electric heater = power / supply voltage = 4500/120 = 37.5 amperes
The circuit breaker rating should be 150% full load current
150% x 37.5 = 56.5 amperes. The nearest one is 60 amperes.